320 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
cent., will be found congregated in the narrow slit 
of light. The source we used was diffused day- 
light, which was admitted through two sheets of 
glass, so that the thermal rays might be considered 
practically excluded. The intensity of the light 
which the Echinoderms are able to perceive may be 
very feeble indeed ; for in our first experiments we 
boarded up the face of the tank with ordinary pine- 
wood, in order to exclude the light over all parts 
of the tank except at one narrow slit between two 
of the boards. On taking down the boards we 
found, indeed, the majority of the specimens in or 
near the slit of light; but we also found a number 
of other specimens gathering all the way along the 
glass face of the tank that was immediately behind 
the pine-boards. On repeating the experiment 
with blackened boards, this was never found to be 
the case; so there can be no doubt that in the first 
experiments the animals were attracted by the 
faint glimmer of the white boards, as illuminated 
by the very small amount of light scattered from 
the narrow slit through a tank, all the other sides 
of which were black slate. Indeed, towards the 
end of the tank, where some of the specimens were 
found, so feeble must have been the intensity of 
this glimmer, that we doubt whether even human 
eyes could have discerned it very distinctly. Owing 
to the prisms at our command not having suid 
dispersive power for the experiments, ntl not wish- 
ing to rely on the uncertain method of employing 
coloured glass, we were unable to ascertain how the 
Echinoderms might be affected by different rays. 
